Polyurethane foam is prepared commercially in the form of large blocks that are subsequently cut into the desired shape for use in the manufacture of various articles that require a foam padding. It is well known that polyurethane foam requires time to cure and develop its full physical properties. In typical polyurethane foam formulations, a polyhydroxy material ("polyol"), water and an organic isocyanate compound are reacted in the presence of catalysts or other additives. Much of the time, a small percentage of terminal isocyanate groups are left unreacted in the foam structure. If the foam is distorted or compressed in this condition, it fails to recover its original dimensions when the distortive or compressive force is released. Normally, the terminal isocyanate groups that are left unreacted in the foam structure will react with the residual water in the foam structure or with the water vapor in the atmosphere over a period of several hours or days, and the foam will ultimately achieve its full physical properties.
As pointed out in the Encyclopedia of Polymer Science and Technology (John Wiley and Sons, New York 1969) in the section on Polyurethanes, polyethers are commercially the most important of the polyols used to prepare polyurethanes. At the present time most of the polyethers used in the production of flexible polyurethane foams are derived from propylene oxide and ethylene oxide. In this preparation, propylene oxide is reacted with glycerol in the presence of a basic catalyst to form a poly(oxypropylene) homopolymer which is further reacted with ethylene oxide to form a block copolymer.
According to the prior art, melamine has been used in both flexible and rigid foams as a fire retardant additive, either alone or in combination with other materials such as silica, alumina, halogenated phosphorus ester compounds, and the like.
For example, British Patent Specification No. 2,094,315 discloses an intumescent, highly resilient polyether urethane foam prepared by reacting a polyether polyol, an organic polyisocyanate, a catalyst, a surface active agent, a blowing agent, an intumescent material, a carbonific element for forming a carbonaceous char by reaction with the acid liberated from the intumescent material, and optionally, a spumific element for generating non-flammable gases which contribute to the intumescence and to a reduction of the effects of flame on the surface of the resulting foam. Melamine is discosed as a suitable spumific element, and the examples show the use of 10 or 20 parts melamine based on 100 parts polyol.
Another example of the use of melamine in flame-resistant flexible polyurethane foams is found in U.S. Pat. No. 4,258,141. These foams generally contain a specific aromatic isocyanate compound, a polyol, flame inhibitors, and blowing agents, with optional additions of chain extenders and other additives. The amount of melamine (or other cyanic acid derivative) ranges from 10 to 70 weight percent, preferably 20 to 50 weight percent, based on the weight of the aromatic polyisocyanates or mixtures of aromatic polyisocyanates.
Melamine has also been used as an additive to the foam forming components of other foams, such as polyester polyurethane foams and rigid foams.
U.S. Pat. No. 4,317,889 discloses flexible, resilient polyester polyurethane foams with substantially improved charforming or intumescent properties, obtained by adding to a conventional polyester polyurethane foam forming reaction mixture at least one melamine derivative, at least one flame retardant, and hydrated alumina. The amount of melamine derivative generally ranges from about 10 to 30 parts by weight based on 100 parts by weight of the polyester polyol.
Also, U.S. Pat. No. 3,897,372 discloses polyurethane foam compositions having flame retardancy and reduced smoke density formed by reacting specific polyisocyanate capped polyoxethylene glycol resin reactants with water. Melamine is added to these foam-forming compositions in an amount of between 1 and 200 parts by weight based on 100 parts by weight of the resin reactant, along with between 50 and 400 parts by weight of aluminum hydrate.
Rigid polyurethane foams which include melamine powder in an amount of between 20 and 100 parts by weight based on the weight of the polyhydroxyl compound are described in U.S. Pat. No. 4,221,875. Also, West German Patent No. 2,348,838 discloses a method for flameproofing synthetic polyurethane materials by adding to a mixture of polyisocyanates, catalysts, polyols, foaming agents, and auxiliary agents, melamine as a flameproofing agent in an amount of between 2.5 and 50% by weight based on the total weight of the reaction mixture.
Rebond polyurethane foam compositions having melamine or urea incorporated therein are described in U.S. Pat. No. 4,385,131. These additives are included in an amount of between about 40 to 100 parts per 100 parts of polyurethane foam chips. The additives and foam chips are joined by a liquid binder to form flame retardant rebond foam articles.
U.S. Pat. No. 3,726,835 discloses that melamine or dicyandiamide can be utilized as a stabilizer for polyurethane prepolymers which are thereafter cured to form elastomeric polymers. In these compositions, 10 parts melamine or dicyandiamide is added to 100 parts prepolymer.
Post-curing of polyether derived foam by exposure to a mixture of water vapor and gaseous ammonia, primary or secondary amines at temperatures of about 50.degree. to 150.degree. F. for a period of at least one minute is disclosed in U.S. Pat. No. 4,537,912. While this process effectively and rapidly cures the foam, it constitutes an additional step beyond those normally used in the foam manufacturing process. This process also requires storage of the foam prior to the postcuring treatment. It is more desirable to cure the foam as it is manufactured to reduce or even eliminate such storage time and to prepare a fully cured foam material which can be immediately shipped to the end users after cutting to the desired shapes.
None of these references disclose the possibility of using melamine or melamine derivatives for rapidly curing polyether polyurethane foam while concommittantly improving the compression set properties.
The present invention provides a one-step foaming and curing process which achieves a rapid and full post cure of the foam so that low compression set values as measured by ASTM standard test D-3574 (Constant Deflection Compression Set Test) are obtained, with the elimination of post curing steps.